1. Field of the Invention
The present invention relates to a magnetic resonance imaging system including a multicoil composed of a plurality of small receiving coils arranged nearby an object under medical examination to pick up magnetic resonance signals (referred to as MR signals, hereinafter) over a wide region of the object under medical examination, and more particularly to a magnetic resonance imaging system which can obtain MR signals of different frequency bands from MR signals picked up by the small receiving coils to improve the S/N ratio.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a partial configuration of a prior-art magnetic resonance imaging system 120, in which a plurality of surface coils 101A to 101C are arranged as a multicoil. In the drawing, the system comprises roughly a probe assembly 100, a data processing unit 105, and a display unit 111. The probe assembly 100 includes a plurality of small receiving coils 101A to 101C arranged on the same plane to pick up MR signals generated from an MR signal source, that is, an object under medical examination (not shown); a plurality of tuning/impedance matching circuits 102A to 102C for tuning the picked up MR signals and matching impedances of the coils with those of the succeeding stage, respectively; a plurality of preamplifiers 103A to 103C for amplifying the output signals of the coils respectively; and an adder 104.
Each coil 101, circuit 102 and preamplifier 103 are connected in series, separately in this order. Therefore, MR signals outputted from the small receiving coils 101A to 101C are given to the adder 104 through the tuning/impedance matching circuits 102A to 102C and the preamplifiers 103A to 103C, respectively. The adder 104 selects the MR signal outputted from any desired single small receiving coil or adds the MR signals outputted from the plural small receiving coils, before transmitting the MR signals to the data processing unit 105.
In this case, when any one of the small receiving coils 101A to 101C is selected, the MR signals are picked up at a small MR image region of the object under medical examination through the selected single small receiving coil. On the other hand, when two or three small receiving coils are selected and combined in any given combination, the MR signals are picked up over a large MR image region of the object under medical examination through the plural combined small receiving coils equivalent to a large receiving coil obtained by connecting the outermost contours of the combined small receiving coils.
The data processing unit 105 includes a filter 106, a detector 107, an A/D converter 108, a memory 109, and an image reconstruction circuit 110. Therefore, MR signals within a predetermined frequency band can be obtained from the MR signals transmitted to the data processing unit 105 through the filter 106, picked up through the detector 107, converted into digital MR signals through the A/D converter 108, and then stored in the memory 109.
These stored MR signal data are given to the image reconstruction circuit 110 for Fourier transformation so that an image of the object to be examined can be reconstructed on the basis of these MR signal data. The reconstructed image data are then given to the display unit (e.g. CRT) 111 to display a predetermined cross-sectional image of the object under medical examination in response to the reconstructed MR image signals. The above-mentioned prior-art MR imaging system is disclosed in U.S. Pat. Nos. 4,825,162, 4,859,947 or 4,975,644, for instance, therefore these documents being incorporated by reference herein.
In general, when a large-sized receiving coil with a large sensitivity range is used to pick up MR signals over a large MR image region or over a voluminous image portion of an object under medical examination, since random noise generated from all over the sensible region of the object to be examined is also picked up by the receiving coils, there exists a problem in that the S/N ratio is reduced and therefore the reconstructed MR image is deteriorated in image quality.
In the case of the above-mentioned multicoil, when the MR signals outputted from a single small receiving coil is processed, the image obtained at a small MR image region is high in S/N ratio and image quality. However, when the MR signals outputted by plural small receiving coils are added by the adder 104 and then processed, although the MR image region (i.e. the visual field of MR image) increases, the image over a large MR image region is low in S/N ratio and image quality. This is because the effective sensitivity range of the receiving coil increases, with the result that the intensity of random noise received from an object under medical examination increase.